Electron-beam furnace with double-coil magnetic beam guidance



CRQSS REFERENCE SEARCH Emu EEE EZE C. W. HAN KS Oct. 1, 1963 ELECTRON-BEAM FURNACE WITH DOUBLE-COIL MAGNETIC BEAM GUIDANCE 2 SheetsSheet 1 Filed May 2'7. 1960 INVENTOR. (#42455 M flAA KJ BY WMX/VW HTTOP/Vi/J Oct. 1, 1963 c. w. HANKS 3,105,275

ELECTRON-BEAM FURNACE WITH DOUBLE-COIL MAGNETIC BEAM GUIDANCE Filed May 27. 1960' 2 Sheets-Sheet 2 United States Patent 3,105,275 ELECTRON-BEAM FURNACE WITH DOUBLE-CQIL MAGNETIC BEAM GUIDANCE Charles W. Hanks, Orinda, Califl, assignor to Staulfer Chemical Company, New York, N.Y., a corporation of Delaware Filed May 27, 1960, Ser. No. 32,217 4 Claims. (Cl. 2257.2)

This invention relates to electron-beam furnaces for heating materials by electron bombardment in a high vacuum, and particularly for melting materials and casting ingots therefrom, with resulting purification, degasification to an exceptionally high degree, and other benefits. An object of the present invention is to provide improved, larger-scale operation for longer uninterrupted periods of time. Other objects and advantages will appear as the description proceeds.

Briefly stated, electron-beam melting-and-casting furnaces include, within a continuously evacuated tank, a container for the molten material, which most commonly has the form of an annular, water-cooled casting mold open at its top and bottom ends. solidified material may be progressively withdrawn through the bottom end of the mold to form a cast ingot of progressively increasing length. An electron gun directs a beam of electrons downward into the open top end of the mold to bombard the material therein and maintain a molten pool of material atop the solidifying ingot. Preferably, melt stock is progressively fed in e.g., from one side of the beam, so that the melt stock is continually melted off as it advances into the electron beam. The so-melted material falls into the mold for continually replenishing the molten material in the pool.

The copending application of Hugh R. Smith, .Tr., Serial No. 32,215, filed May 27, 1960, entitled Electron Beam Furnace With Magnetically Guided Beam, which is assigned to the same assignee as the present application, discloses and claims an improved electron-beam furnace comprising an annular electromagnet winding extending coaxially around the annular mold. The magnetic field of this winding guides electrons from the gun into the open top end of the mold, even though gaseous material evolved from melting and molten material may provide a copious supply of ions within the space traversed by the beam.

The present invention is a further improvement, providing significant further benefits and advantages, including the following: greater isolation of the electron gun from the evolved gaseous matter, resulting in a great reduction in the frequency of electrical shorts at the gun, greatly increased gun life, and much longer periods of uninterrupted operation; better control of the melting surface of the melt stock, the melting plane being approximately vertical and very sharply defined; a sharper focus of the electron beam on the molten pool within the mold, even in the present of dense plasma formed by ionization of the evolved gaseous matter; more flexibility of adjustment of the beam pattern, enabling the operator to compensate more readily for any unfavorable circumstances which may arise during a melting operation-cg, the beam pattern can be readjusted to cut off any horns that may develop from the melting surface of the melt stock, also to permit continued operation even with part of the electron-gun filament shorted.

In accordance with the present invention, two vertically spaced, annular electromagnet windings are employed, one extending around the annular mold and the other vertically spaced above the open top end of the mold. The two windings are in vertical, coaxial alignment, and are energized in additive magnetic flux relation. These two windings, in combination, produce a magnetic field having flux lines that converge inwardly and downwardly "ice through the center opening of the upper winding into the open top end of the casting mold, the flux lines being approximately vertical within the space between the two windings. The electron gun is positioned above the upper winding and provides a hollow, conelike electron beam converging inwardly and downwardly parallel to the magnetic flux lines. The magnetic field guides the beam through the center opening of the upper winding into the open top end of the mold. Melt stock preferably is fed horizontally inward between the two vertically spaced windings. The energizing currents of the two windings are individually adjustable by means of rheostats or the like, and this provides greater flexibility in the adjustment and control of the electron-beam pattern.

The foregoing and other aspects of the invention may be understood better from the following illustrative description and the accompanying drawings.

FIG. 1 of the drawings is a highly schematic, vertical section of an improved electron-beam furnace.

FIG. 2 is a fragmentary schematic, vertical section of the same furnace, drawn to a somewhat larger scale, showing typical magnetic fiux lines and electron trajectories.

Referring to the drawings, an annular, copper mold 1, with its axis vertical, has open upper and lower ends and is provided with a water jacket 2 through which water or other coolant is continuously circulated by conventional means (not shown), whereby the mold is cooled to solidify molten material therein. Other parts of the furnace may also be water-wooled, as desired, such being accomplished by obvious means requiring no description. The solidified material may be progressively withdrawn through the open bottom end of mold 1 to form a cast ingot 3 of progressively increasing length, which may be cut off from time to time as desired. Progressive withdrawal of the cast ingot is accomplished, for example, by means of rollers 4 driven by an electric motor 5.

A first annular electromagnet winding 6 extends coaxially around mold 1, as shown, and has a vertical axis concentric with the open top of the mold. Preferably, this winding is protected by an inner sheath 7 of insulation and an outer sheath 8 of metal. Wires 9 and 10 connect Winding 6 to a DC. power supply 11 in series with a rheostat 12, whereby the winding 6 is supplied with energizing direct current of adjustable magnitude. A second electromagnet winding 13 is vertically spaced above the first winding, in vertical, coaxial alignment with the first winding 6 and the annular mold 1. Preferably, winding 13 is protected by an inner sheath 14 of insulation and an outer sheath 15 of metal. Wires 16 and 17 connect winding 13 to DC. power supply 11 in series with a rheostat 18, whereby winding 13 is supplied with energizing direct current of adjustable magnitude.

The two windings 6 and 13 are energized in additive magnetic flux relation, and the energizing currents supplied to the two windings are individually adjustable by means of rheostats 12 and 18. The so-energized windings, in combination, produce a magnetic fiel-d of adjustable strength having a vertical axis of symmetry and having magnetic flux lines that converge inwardly and downwardly through winding 13 into the open top end of mold 1, as represented by broken lines 19, FIG. 2. The magnetic flux lines are approximately vertical in the space between the two windings, but can be adjusted to some extent in position and orientation by adjusting the relative magnitudes of the two energizing currents. The furnace is usually operated with the ampere-turns energization of windings 6 and 13 approximately equal.

An annular electron gun is vertically aligned above winding 13. Preferably, the distance between windings 6 and 13 is about the same as the diameter of the mold and the spacing between winding 13 and the electron gun is relatively large. In its preferred form, the electron gun comprises an annular thermionic cathode 29, most commonly made from a horizontal loop of tungsten wire, connected through leads 21 and 22 and a transformer 23 to an alternating-current supply 24, which supplies alternating current through wire for heating the same to thermionic-emission temperature. An accelerating electrode 25 is closely spaced below cathode 20, and a focusing electrode 26 is closely spaced above cathode Ztl, as shown. Electrical connections are provided for maintaining accelerating electrode 25 at substantially the same electric potential as mold 1, preferably ground potential. This is indicated schematically in the drawing by the ground connection symbols at 27 and 28. Cathode 2d and focusing electrode 26 are maintained at substantial negative potentials, commonly 5,000 to 15,000 volts, relative to the accelerating electrode. This is accomplished, for example, by means of connection 29 between electrode 26 and lead 22 and connection 39 between lead 22 and the negative terminal of a high-voltage D.C. supply 31.

The overall design of the electron gun may be similar to that described in the copending patent application of Charles W. Hanks, Serial Number 818,306, filed June 5, 1959, and assigned to the same assignee as the present application. In the present furnace employing a magnetic field to guide the electron beam, the cathode, accelerating electrode, and focusing electrode are shaped and aligned to direct electrons downwardly and inwardly, parallel to the converging magnetic lines of force, forming a hollow, conelike, converging beam, and thereafter the magnetic field plays a significant part in focusing and guiding the electron beam through the central opening of winding 13 and into the open top end of mold 1.

A horizontal feed trough 32 extends inwardly above mol'd 1 between the vertically spaced windings 6 and 13. Discharge end 32 is adjacent to the open top end of the mold. This feed trough (and thereby its content) is electrically grounded as indicated by the conventional symbol at 33. Melt stock 34 in any convenient form, e.g., rods, bars, blocks of compacted powder, etc., is fed through trough 32 into one side of the electron beam entering mold 1. Feed mechanism is symbolized by rollers 35 driven by an electric motor 36. As the melt stock emerges from the discharge end of the feed trough, it is bombarded and melted away by the electron beam. The so-melted material falls into the open top end of mold 1, for continually replenishing a pool 3' of molten material which rests on top of cast ingot 3 and is supported within a skull or depression that forms autom'atically in the solidified material.

The second annular winding 13 significantly improves the magnetic guidance, appreciably improves the operation of the furnace, and permits operation on a larger and more economical scale for longer periods of time without interruption. In the space between the two windings, the magnetic flux lines and the electron trajectories are approximately vertical, and the outer edge of the electron beam is substantially vertical and very sharply defined. As a result, the melt stock fed in through trough 32 melts off along a very sharply defined, and precisely located, vertical melting plane. Evolved gaseous matter issuing from this melting plane, often erratically and at considerable velocity, travels for the most part horizontally rather than upwardly, and therefore is directed away from, rather than toward, the electron gun.

Much of the evolved gaseous matter will condense readily upon any nearby cool surface, and the metal sheaths surrounding the two electromagnet windings serve as elficient collectors of this gaseous matter. This action may be enhanced by cooling the metal sheaths 8 and 15, as by continually circulating water or other coolant through the metal pipes 37 and 38 attached there to. In this way vapors are condensed at places where the buildup of deposited matter is of relatively little consequence, and the vapors are kept away from the electron gun where such buildup of deposited matter is most disadvantageous. The matter deposited on sheaths 8 and 15 can be scraped off at relatively infrequent intervals when the furnace is shut down for routine maintenance.

Provision of a vertical melting plane has other advantages: control of the melting rate is better; the melt stock casts a smaller shadow upon the molten pool, i.e., a smaller area on the pool surface that is shielded from electron bombardment by the overhanging melt stock; and there is little chance for solid bridges to form between the end of the melt stock and the ingot within the mold. Even if such bridges should tend to form, they can be cut away by adjusting the field pattern with rheostats 12 and 18.

Even dense plasmas created by the ionization of relatively large quantities of gaseous matter evolved from the melting material have very little effect upon the electron beam pattern when double-coil magnetic guidance is employed in accordance with this invention. This may be because the dense plasma is largely confined to the space between the two windings, in which the electron trajectories are substantially vertical and closely parallel to the magnetic flux lines. Electrons traveling at 'high velocity along such straight trajectories are not easily deflected or diverted, and therefore even a highly conductive plasma in this region has little efiect upon the beam pattern.

'In operation, the energizing current supplied to winding 13 is adjusted by means of rheostat 18 to obtain the desired confinement of the electron beam, particularly the desired cross-sectional area of the beam passing through the central opening of winding 13. The energizing current supplied to winding 6 is adjusted by means of rheostat 12 to obtain the desired beam pattern upon the surface of molten pool 3'. There is some interdependence between these two adjustments, and in practice rheostats 12 and 18 may be adjusted substantially concurrently for both purposes, but the principal effects of the two adjustments are as described.

It has been found that these two independent adjustments of the electromagnet energizing currents afford great versatility and precision in the control of beam pattern under widely varying and sometimes adverse conditionsfor example, these adjustments permit the desired beam pattern to be maintained despite wide variations in melting rates, the quantity of gaseous matter evolved from the melting material, changes in the kind of material being processed, and the like. Furthermore, should horns or other projections appear on the melting plane, or should there be a tendency for solid bridges to develop between the melt stock and the cast ingot, or if any portion of the molten pool should become too cool and tend to solidify, corrective action is easily provided by readjusting the beam pattern by means of the two rheostats.

It is even possible by this means to continue operation with a portion of the electron gun filament shorted, e.g., by material deposited from condensating vapors, so that segments of the conelike electron beam are missing. The magnetic field can often be adjusted to spread what remains of the beam over the molten pool so that operations can be continued without interruption. This is quite important because it enables the casting of an ingot to be completed even if a failure of the type described should occur midway in the casting of an ingot. To interrupt the operation at such time would be disadvantageous, not only because of time lost due to the interruption, but because of the possibility that the ingot may contain a flaw at the point where interruption occurs.

In FIG. 2, the volume occupied by the electron beam is represented by the shading between lines 39 and 40 and between lines 4-1 and 42. It will be noted that the beam -is everywhere substantially parallel to the magnetic flux lines that guide the beam.

In FIG. 1, there is schematically shown a vacuum tank 43 which encloses the casting mold, the electron gun, and associated parts. Tank 43 is continually evacuated to a high vacuum, preferably .one micron of mercury absolute pressure or less, by connection of the tank through a large-area duct 44 to high-capacity vacuum pumps 45. Appropriate air locks (not shown) may be provided as desired for the introduction of melt stock, the removal of ingots, the replacement of electron guns, and the like.

It will be understood that the specific embodiment illustrated is but one example of how this invention may be carried out, and that numerous changes and modifications are possible without departing from the inventive principles herein disclosed.

What is claimed is:

"1. An electron-beam furnace comprising a container for molten material, said container having an open top, tWo coaxial electrom-agnet windings, one of said windings extending around and at least partly below the upper end of the container, the other winding being vertically spaced above the open top of said container, means for supplying energizing current to each of said windings, the so-energized windings jointly providing a magnetic field having flux lines converging into said container through its open top, an electron gun laterally displaced from above the container, the gun being aligned to project a beam of electrons along said converging flux lines into said container, a vacuum tank enclosing at least the windings and the space between said gun and said container, and means for evacuating said tank continuously.

2. A11 electron-beam furnace comprising an annular container for molten material, said container having an open top end, two vertically spaced, annular, electromagnet windings disposed in vertical, coaxial alignment, one of said windings extending coaxially around and at least partly below the upper end of the container, the other winding being vertically spaced above the open top end of the container, direct-current supply means connected to energize said windings in additive magnetic flux relation, the so-energized windings jointly providing a magnetic field having flux lines extending and converging downwardly through the upper annular winding into the open top end of said container, an electron gun disposed above said upper winding and laterally displaced from above the container, the gun being aligned to project a beam of electrons downwardly along said converging ilux lines through said annular upper winding into the open top of said container, means for feeding material to be melted between said vertically spaced windings into one side of said beam of electrons, whereby the fed-in material is melted by the electron beam and falls into said container, a vacuum tank enclosing said wind ings, container and gun and the space therebetween, and means for evacuating said tank continuously.

3. An electron-beam furnace as in claim 2, said electron gun comprising an annular thermionic cathode vertically aligned above said upper winding, an accelerating electrode closely spaced below said cathode, a focusing electrode spaced above said cathode, means maintaining said accelerating electrode and container at substantially equal electric potentials, and means maintaining said cathode and focusing electrode at substantial negative electric potentials relative to the accelerating electrode, said focusing electrode and cathode and accelerating electrode being shaped and aligned to form a hollow conelike electron beam directed downwardly and converging inwardly, being everywhere substantially parallel to said converging magnetic flux lines.

4. An electron-beam furnace for melting materials and casting ingots therefrom in a high vacuum, comprising an annular, metal mold having a vertical axis and having open top and bottom ends, means for continually cooling said mold to solidify molten material therein, means for progressively withdrawing the solidified material through the bottom end of the mold as an ingot of progressively increasing length, a first annular electomagnet winding extending coaxially around said mold and below said open top end, a second annular electromagnet winding spaced vertically above the open top end of said mold, said second winding being in vertical, coaxial alignment with said first winding and mold, directcurrent supply means connected to energize said first and second windings in additive magnetic fiux relation, the soenergized windings jointly providing a magnetic field having flux lines extending and converging downwardly through said second annular winding into the open top end of said mold, means for individually adjusting the energizing currents of said windings, an annular electron gun vertically aligned above said second winding, said gun comprising a thermionic cathode consisting of a horizontal loop of wire and connections for supplying heating current therethrough, the loop being of substantially greater diameter than the mold and coaxially disposed around the vertical axis passing through the mold, an accelerating electrode closely spaced below said cathode, a focusing electrode closely spaced above said cathode, means for maintaining said accelerating electrode at substantially the same electric potential as said mold, and means for maintaining said cathode and focusing electrode at substantial negative potentials relative to the accelerating electrode, said focusing electrode and cathode and accelerating electrode being shaped and aligned to form a hollow conelike electron beam directed downwardly and converging inwardly, being everywhere substantially parallel to said converging magnetic flux lines, extending through said second winding into the open top end of said mold, a vacuum tank enclosing said windings, electron gun and said mold and the space therebetween, means for continuously evacuating said tank to a high vacuum, a substantially horizontal feed trough extending inwardly toward and having a discharge end closely adjacent to the open top end of said mold and between the two windings, means for maintaining said trough at substantially the same electric potential as said mold, and means for feeding material to be melted through said trough into one side of the electron beam entering the mold, whereby the material emerging from the discharge end of the feed trough is melted away by the electron beam and the so-melted material falls into the open top end of the mold.

References Cited in the file of this patent UNITED STATES PATENTS 2,321,886 Anderson June 15, 1943 2,423,729 Ruhle July 8, 1947 2,880,483 Hanks et al. Apr. 7, 1959 2,963,530 Hanks et al. Dec. 6, 1960 3,005,859 Candidus Oct. 24, 1961 

1. ELECTRON-BEAM FURNACE COMPRISING A CONTAINER FOR MOLTEN MATERIAL, SAID CONTAINER HAVING AN OPEN TOP, TWO COAXIAL ELECTROMAGNET WINDINGS, ONE OF SAID WINDINGS EXTENDING AROUND AND AT LEAST PARTLY BELOW THE UPPER END OF THE CONTAINER, THE OTHER WINDING BEING VERTICALLY SPACED ABOVE THE OPEN TOP OF SAID CONTAINER, MEANS FOR SUPPLYING ENERGIZING CURRENT TO EACH OF SAID WINDINGS, THE SO-ENERGIZED WINDINGS JOINTLY PROVIDING A MAGNETIC FIELD HAVING FLUX LINES CONVERGING INTO SAID CONTAINER THROUGH ITS OPEN TOP, AN ELECTRON GUN LATERALLY DISPLACED FROM ABOVE THE CONTAINER, THE GUN BEING ALIGNED TO PROJECT A BEAM OF ELECTRONS ALONG SAID CONVERGING FLUX LINES INTO SAID CONTAINER, A VACUUM TANK ENCLOSING AT LEAST THE WINDINGS AND THE SPACE BETWEEN SAID GUN AND SAID CONTAINER AND MEANS FOR EVACUATING SAID TANK CONTINUOUSLY. 